1. Field of the Invention
The present invention relates to a motor-equipped reduction gear device with a so-called pre-stage.
2. Description of the Prior Art
As shown in FIG. 3, normally a motor-equipped reduction gear device (a so-called geared motor) is composed of a motor 2, a joint cover 4 and a gear box 6. A pinion 10 is mounted on the tip of the motor shaft 8 of the motor 2. The pinion 10 meshes with a first-stage gear 15 in the gear box 6. The joint cover 4 is mounted in order to cover and protect the portion where the motor 2 and the gear box 6 are linked.
Normally the gear box 6 comprises plural (1-3 or 4) built-in gear pairs. These gear pairs provide specified speed reduction ratios.
Cylindrical gears with parallel axis, bevel gears, worm gears or planetary gears are examples of these gear pairs.
For example, the speed reduction ratio which can be achieved with two gear pairs consisting of cylindrical reduction gear with parallel axes (a so-called in-line type) is normally 25 (=5.times.5) and approximately 70 (=10.times.7) at maximum.
Accordingly, in order to obtain a bigger speed reduction ratio, it is necessary to provide one or two additional gear pairs between the motor 2 and the gear box 6, as shown in FIG. 4. In general, an additional speed reduction mechanism like this provided between the motor 2 and the gear box 6 is called a pre-stage.
In FIG. 4, a pre-stage with one gear pair is shown. That is, the pinion 10 mounted on the tip of the motor shaft 8 meshes with the gear 12. The pinion 10 and the gear 12 cause a speed reduction of the motor rotation before the motor rotation is transmitted to the gear box 6. The reduced rotation is transmitted through the pinion 14 to the first-stage gear 15 in the gear box 6.
Reference numerals 40, 42 and 44 indicate bearings.
However, if such a pre-stage is provided, the additional gear 12 and pinion 14 are necessary. The gear 12 and pinion 14 in turn require a supporting mechanism, which causes the problem that an additional pre-stage cover 16 must be provided between the joint cover 4 of the motor and the gear box 6.
That is, in addition to the joint cover 4, the pre-stage cover 16 must be provided between the motor 2 and the gear box 6 in order to obtain bigger speed reduction ratio than the reduction ratio which originally the conventional geared motor has. In short, there is the problem that two types of covers have to be mounted.
In general, the pro-stage cover 16 is fixed to the gear box 6 with bolts 18 and to the joint cover 4 with bolts 20. This means that number of bolt connections increase, which in turn involves the problem like that it takes longer assemble time and appearance and shape of geared motor are largely restricted.
Furthermore, the area necessary for mounting the geared motor with a pre-stage on a matcing machine increases, especially the axial length becomes longer, which causes many problems such as losing of compactness, increasing of weight and increasing a chance of easily arising difficulties for assembly.
Also, since the motor 2 and the gear box 6 are linked by two members, the machining errors of each member are added up so that the accuracy of the center distance and the parallelism between the pinion 10 and the gear 12 decrease. This causes non-uniform contact between the tooth surfaces of the gear, which is obstructive to the smooth rotation of the gear and which reduces the life of the gear.
Furthermore, the large number of necessary parts causes an increase in costs and a decrease in the rigidity as compared to a one-body product. Also, the large number of faces which have to be connected to each other causes the problem of increasing a chance of easy oil leakage.